Within modern magnetic disk drives (HDD), a disk is rotated at high speed and a Head Gimbal Assembly (HGA) is driven at high speed in response to demands for high capacity, high recording density and high-speed access. Accordingly, considerable air disturbance occurs, which causes vibration at the disk or the HGA. The air disturbance vibration will be a huge obstacle when positioning a head to data recorded in high density on the disk. The air disturbance occurs at random, and it is difficult to predict the magnitude or a cycle thereof, therefore, the rapid and accurate positioning will be complicated and difficult. In addition, the air disturbance will be a factor of noise, and also will be a factor impairing the quietness of the drive.
As a problem caused by action of air in the drive due to the high-speed rotation, there is the increase of power consumption in addition to the above. When the disk is rotated at high-speed, air in the vicinity of the disk is also involved and rotated. On the other hand, air which is distant from the disk is still, therefore, shearing force is generated therebetween, which will be a load to stop disk rotation. This is called as windage loss, which increases as the rotation speed becomes high. In order to rotate at a high speed against the windage loss, a motor requires large output, therefore, large electric power is necessary.
There was an idea in which air disturbance and windage loss are reduced by sealing gas which has lower density than air, instead of air, in a sealed magnetic disk drive, focusing attention on the fact that the disturbance and windage loss are relative to gas density in the drive.
As low-density gases, hydrogen, helium and the like can be used. Considering actual use, helium is most suitable, which is highly effective, stable and safe. In the magnetic disk drive in which helium gas is sealed therein, the above problems can be solved as well as rapid and accurate positioning control, low power consumption, and quietness can be realized.
However, helium has extremely small molecules and a large diffusion coefficient, therefore, there is a problem that a casing used for the normal magnetic disk drive has low-sealing performance and helium leaks easily during normal use.
In order to allow low-density gas such as helium which leaks easily to be sealed, for example, a sealing structure disclosed in U.S. Patent Publication No. 2005-0068666 (“Patent Document 1”) is proposed. FIG. 9 is a cross-sectional view showing a casing structure of a magnetic disk drive disclosed in Patent Document 1. A casing 100 includes a base 120 and a cover 110 jointed to upper portions of a sidewall of the base 120, and a head disk assembly (HDA) 101 is housed in a casing interior 102. In order to seal helium in the casing interior 102, the cover 110 is joined in an environment of helium gas, and at the same time of the joining, the casing interior 102 becomes a sealed magnetic disk drive which is filled with helium.
As a point in which the risk of leakage of helium gas in the casing is high, a joint portion 105 between the upper portion of the sidewall of the base 120 and the cover 110 is cited. In order to seal the joint portion 105 completely, the cover 110 is laser-welded on the upper portions of the sidewall of the base 120.
Concerning the base and the cover, in the light of durability, reliability and costs, a base molded by an aluminum die casting, and an aluminum cover formed by pressing or cutting are selected.
In the related arts, since the base and the cover are jointed by the laser welding, considerable welding area must be secured. Accordingly, it is required that the base sidewall of the magnetic disk drive is sufficiently high and wide. However, when aiming to miniaturize the magnetic disk drive, it is difficult to apply a structure in which the base sidewall is sufficiently high and wide. In the magnetic disk drive in which helium is sealed, a method is considered, for performing an inspection in a state of sealing helium and performing repair operation, in which an inner cover is attached temporarily on the base and helium gas is sealed in the casing interior until the inspection of the HDA is finished, and after the inspection is finished, an outer cover is joined by a welding and the like to perform complete sealing. However, when aiming to miniaturize the magnetic disk drive, it is difficult to apply the double cover structure as the above method because the height direction of the base sidewall becomes large.